This invention relates generally to rotary drum screens commonly known as microscreens used for the filtration of liquids in conjunction with water treatment, sewage treatment, and industrial waste treatment. In particular, this invention relates to an improved microscreen drum support apparatus which includes the use of polymeric support wheels.
Microscreens of the type involved herein are well known in the art, for U.S. Pat. No. 4,038,187 is directed to improvements in such a drum. The construction of such devices may be easily understood by referring to FIG. 1. As shown, microscreens comprise a drum 10 having a closed end 11, an open end 12, a central axis 13, a radial outer surface 14, a plurality of microscreen grids 15 mounted on the radial outer surface 14 of the drum 10, means 16 for introducing the liquid to be filtered into the interior of the drum 10, means for rotating the drum about its central axis 13, a backwash header 20 mounted above the radial outer surface of the drum 10, a backwash receiving trough (not shown) mounted within the drum 10 for collecting solids washed off the microscreen grids 15 by the backwash header 20, and means (not shown) for controlling the speed at which the rotating means rotates the drum 10 and/or for controlling the output of the backwash header 20 in response to the accumulation of solids on the microscreen grids 15.
The microscreen drum 10 is mounted in a rectangular tank 5 and is rotatably supported on a frame 30 by a journal bearing 32 on the central axis 13 at the closed end 11 and a plurality of support wheels 36 mounted at the opened end 12 of drum 11 and constructed and arranged to rotate on the inner peripheral rim 34 of drum 10. The axial bore 38 of wheels 36 rotates on bearings 39 rotating on axle 46 attached to truck 44 which pivotally engages elongate connecting member 42 attached to frame 30 at base 41.
Wastewater enters the feed well 17 through inlet 16 where it accumulates until it exceeds the height of the inner weir 19. The water then rushes into the interior of rotating drum 10 and its plurality of microscreen grids 15 which trap suspended particulate materials contained in the water. The trapped particulates are rotatably transported to an upper region where they are flushed from the grids 15 into a collection trough (not shown) by a backwash header 20. The outer edge of the open end 12 of the drum 10 is sealed so that incoming water can only get to the exit weir 18 through grids 15. The filtered water flows through the grid 15 and, once it exceeds the height of the exit weir 18, spills into the collection chamber 21.
Microscreens are subject to a certain degree of misalignment between the drum 10 and frame 30 as a result of the hydraulic thrust force of incoming water against the closed end 11 of drum 10. This force tends to push the drum 10 longitudinally on its central axis 13 toward the exit end of the apparatus. Misalignment of the microscreen drum and frame also results from often imprecise on-site installation practices. Although a certain amount of misalignment can be accommodated for by the design of the journal bearing 32, the need for a freely rotatable drum makes this misalignment a chronic condition. Consequently, microscreen support wheels must be adaptable to a certain degree of lateral load shifting as they rotatably support the open end 12 of the rotating drum 10.
In addition, microscreen wheels must be able to withstand loading forces from the weight of both the drum and the wastewater contained therein, which are of the magnitude of 7,500 pounds per set of two 12 inch diameter wheels.
Thus, as a result of the inherent misalignment and significant loading due to their relatively small diameters, microscreen support wheels must also be able to perform their principal role of drum support in many instances when the drum is only in partial contact with the tread surface of the wheel. The wheels must be designed so that this excessive edge loading does not cause the resulting imbalanced loads to be transferred eccentrically to hub bearings and wheel supports, which function best when the drum loading force is directed over the center of the wheel.
Conventional steel microscreen support wheels have such a high coefficient of friction that when misalignment occurs, the microscreen drum 10 jumps jerkedly back and forth along the longitudinal central axis, creating excessive vibration, noise and impact loads which decrease the usable life of the drum 10.
Furthermore, steel microscreen support wheels decay fairly rapidly in the highly corrosive environment of a wastewater treatment facility, and their frequent replacement significantly increases the cost of operation for microscreen systems.
Conventional polymeric wheels and casters have been found to be unsuitable to microscreen applications because their combination of structural geometry and material composition render them unable to withstand both the tremendous support loads and the significant rotational edge loading inherent in microscreen operation.
Thus, there is a need for a corrosion-resistant microscreen support wheel with a tread surface having a relatively low coefficient of friction which will cause negligible drum vibration yet allow for drum slippage, as well as being able to withstand a significant edge loading while causing minimal load transfer of imbalanced loads to the wheel hub and wheel supports.
It is therefore a principal objective of the present invention to provide a microscreen support wheel which has a tread surface having a coefficient of friction substantially lower than that of steel-on-steel (0.57-0.80) which will allow the rotating drum to readily slide across that tread surface.
It is a further objective of the present invention to provide a microscreen support wheel able to withstand excessive loading in the order of 3,500 to 5,000 pounds, even when that load is directed substantially to the edge of that wheel.
It is a still further objective of the present invention to provide a microscreen support wheel which is resistant to the corrosive environment of a wastewater treatment facility.
It is another objective of the present invention to provide a microscreen support wheel which generates a minimal amount of operational vibration and noise.